Printing systems include a printing device to impart an image on a substrate media. In direct printing systems, a print head is disposed adjacent the media and an image is directly transferred onto the media. One type of direct printing is a solid inkjet (“SIJ”) system. A key critical dimension associated with the SIJ printing process is the small gap between the ink jet heads and the receiving media. This gap typically is on the order of 0.5 mm, and must be tightly controlled to maintain accurate drop placement which in turn results in acceptable image quality. In the case of web-based media handling systems, one can maintain this gap relatively easily with the proper geometry and web tension. However, for cut-sheet SIJ systems, the sheet edges pose a problem as the edges can be lifted up from a media transport, for example, electrostatic, vacuum, etc., due to curl. If these edges come into contact with the print heads during operation, damage to the jets could occur. As the print heads are expensive, this scenario would negatively impact run cost.
Solutions to this problem have included using a long range sensor which is mounted across the media path and used to detect the entire cross process length of the sheet in real-time at the inspection zone. The long range sensor can be a transmitter-receiver pair, retro-reflective type, or an array/curtain type. However, experience with these methods has revealed difficulties in obtaining parallelism to the transport zone, difficulties in transmitter/receiver pair alignment, etc. Also, secondary reflections due to beam parallelism error and beam divergence were major sources of noise.
Therefore, it is desirable to provide an apparatus which can reliably determine excessive media height to protect to contact sensitive devices.